According to a conventional configuration, a tower crane comprises a vertical mast, a jib, carried by the mast and azimuth orientable about the mast along a movement called orientation movement, and a carriage which is movably mounted in radial translation along said jib thus performing a movement called distribution movement. The carriage carries a hook, suspended from the carriage by a cable whose length is modifiable by means of a winch that commands thus the vertical movement of said hook, called lifting movement. The load is mechanically coupled to the hook by means of slings.
It is known to provide a crane with an onboard system configured to block/prohibit the execution, by the drive members of the crane, of commands produced according to the instructions of the crane operator that are likely to cause movements considered as inappropriate or which could create undesired results.
The transition phase, during which the load placed on the ground is raised in the air, requires special attention. Thus, in the case of a conventional lifting, the crane operator ensures that the liftoff of the load is carried out at reduced speed. In the event that the value of the measured load exceeds a determined threshold, the system is configured to block the lifting movement before said load leaves the ground. The command control system may further include, as described in the patent document U.S. Pat. No. 8,708,170, means for limiting the oscillations observed during the lifting movement, in particular by detecting the passage of the static state where the load is placed on the ground in the suspended state, and by limiting the speed of the lifting movement during the transition between said states.
However, no known command control system includes means to prohibit the liftoff of the load placed on the ground, if the slings have not been stretched prior to this operation. Indeed, the latter provide that a ground operator attaches the relaxed slings to the load and to the hook of the crane. Then, at low speed, the crane operator performs a lifting movement to stretch the slings, while ensuring however that the load does not lift off from the ground. The ground operator may then verify proper slinging and balancing of the load while on the ground. If the verification proves successful, the crane operator can then start a lifting movement to raise the load in the air. Only once the load is in the air, the crane operator can increase the speed of the lifting movement.
Since the known systems are not configured to detect a sudden lifting of the load attached to the hook with relaxed slings, the load can therefore be raised in the air, although it exceeds the maximum permissible load. Indeed, the speed of the lifting movement being high and the slings being relaxed, it is possible that the measurement of the value of the load can be carried out only once the load is in the air. These manipulations can therefore cause the crane to enter an undesired condition, or damage some components, such as the cable, slings, lifting lug, portion of carriage, portion of block, jib, and other similar components subjected to stress during a lifting operation,
That is why there is still a need for means capable of detecting, in order to prohibit them, the movements of lifting in the air a load placed on the ground, if the slings mechanically coupled to the load and to the hook are not stretched.